(a) Field of the Invention
The present invention relates to a titania sol, a method of preparing the same, and a coating composition including the same, and more specifically to a titania sol that can be directly used as a middle or high refractive filler on glasses, industrial safety glasses, or goggles for leisure that require transparency, so as to prepare a coating layer easily, a method of preparing the same, and a composition for coating including the same.
(b) Description of the Related Art
Titania has the highest refractive index among metal oxides, and it is excellent in terms of whiteness and dyeability. Furthermore, titania can be dispersed in a solution with a relatively accurate particle size. Therefore, it is used in various fields such as for an electronic material, a Braun tube of a TV, a coating material for a welding rod, etc., as well as for paint, ink, plastic, paper, a filler for controlling gloss of rubber and fiber, and to enhance durability of chemical fibers and synthetic fibers.
Generally, titania is classified as anatase and rutile according to its crystal form. The anatase and the rutile show different physical properties owing to their different crystal structures. Specifically, the refractive index of the rutile titania, which is 2.72, is higher than the refractive index of the anatase titania, which is 2.52, and it is difficult to make it into a spherical form of a several-nanometer size.
The typical application of the titania to the various coating layers is done by dry coating method including deposition and wet coating using a coating solution. The coating solution for wet coating is prepared in the form of titania sol wherein titania is dispersed homogeneously in a solvent. However, as known, it is difficult to make the titania into a sol because the titania is chemically stable and poorly soluble to an acid, an alkali, water, or an organic solvent, except for fluoric acid, heated concentrated sulfuric acid, and a molten alkali salt. Furthermore, there are many difficulties in preparing a hard coating composition, because the titania does not react with a high reactive gas such as sulfur trioxide (SO3), chlorine gas, etc., under normal temperature and normal pressure.
Therefore, methods of preparing the titania sol through sol-gel reaction have been suggested to have more stable dispersity and ease of application to a coating composition. The sol-gel reaction has a merit such that the titania sol is easily prepared by hydrolysis reaction of a metal halide or an alkoxide.
Korean Patent Publication No. 2001-0028286 discloses a method of preparing the water-dispersed titania sol by sol-gel reaction under normal pressure. The titania sol may be applied to a coating layer, but the solid content is too low to be used as a filler for a high refractive hard coating layer. Furthermore, when the water is removed in order to increase the solid content, there are some problems that aggregations of the particles occur and the particles are enlarged.
Korean Patent Publication No. 2004-0100732 discloses a method of preparing the titania particles by sol-gel reaction under high pressure. It is possible to produce stably dispersed titania nanoparticles by the method. However, it is undesirable to be applied to a real process, because the sol-gel reaction is conducted under a relatively high temperature and pressure, aggregations of the particles occur when re-dispersing them into a solvent after drying the prepared titania nanoparticles, and the solid content must be low in order to make a stable sol.
Korean Patent Publication No. 2002-0043133 discloses a method of preparing highly crystalline and dispersive photo-catalyst of an anatase-type titania sol by hydrothermal treatment. Although the solid content of the titania sol prepared therefrom is relatively high, approaching 10 wt %, the publication mentions that precipitation and gelation occur when the solid content is over the range. Furthermore, although the publication mentions that the solid content may be increased to up to 20 wt % by removing the solvent with distillation under reduced pressure, there is a problem in that the particles are enlarged and aggregations occur.